The present invention relates generally to the field of applying surface diffusion coatings to industrial parts and in particular to a new and useful composition and process for applying uniformly thick diffusion coatings on 400-series stainless steel components.
Chromizing is a process of producing a chromium diffusion coating on ferrous-base components to improve corrosion resistance, especially at elevated temperatures. Chromizing was developed to produce an integral protective surface coating on components exposed to extreme conditions for the purpose of enhancing their usable life. One common application of chromized parts is in power generation, such as for boiler waterwall panels and studs.
Many different processes for chromizing components with surface diffusion coatings of chromium and chromium-silicon have been developed, patented and commercialized successfully.
U.S. Pat. Nos. 5,135,501 and 5,041,309, for example, teach a xe2x80x9cblanketxe2x80x9d process in which an inert refractory container is coated with a slurry of a diffusion composition. The composition may contain chromium as the diffusion element. The container is then inserted within or placed on a workpiece that will be coated with the diffusion coating, and heat treated to cause the diffusion to take place. The container is then removed to leave the chromized workpiece. Commonly, a spun alumnia-silica fiber paper or blanket is used as the inert container.
U.S. Pat. No. 5,912,050 discloses a process for chromizing many small parts in layers within a retort. The parts are placed within the retort on a sheet of refractory felt paper, covered with a diffusion coating slurry that is allowed to dry prior to heating of the retort, while additional layers of felt paper, parts and slurry are added to the retort. The retort is sealed and heated, causing the diffusion coating to adhere to the parts covered in the slurry.
The disclosures of U.S. Pat. Nos. 4,904,501 and 5,135,777 and 5,041,309 and 5,912,050 are hereby incorporated in their entirety for their discussions of different diffusion coating processes.
Chromizing and chrome-siliconizing processes have been shown to work well on many different types of steels, including T2, T22, T23, 1010, 1018, T11, 178A, T91, 304 stainless steel, 309H stainless steel and 347H stainless steel.
However, 400-series stainless steels have been notoriously difficult to apply diffusion coatings to in useful thicknesses and continuities. Conventional diffusion methods typically result in porous chromium diffusions of only 1-2 mils thick and having many discontinuities in the coating on 400-series stainless steels. These problems are especially evident with 430 stainless steel components.
Diffusion coatings that are so thin and porous with discontinuities are effectively useless for their intended purpose of protecting the coated component.
It is an object of the present invention to overcome the difficulties associated with prior diffusion coating processes applied to 400-series stainless steels.
It is a further object of the invention to provide a method for applying a uniform, relatively thick diffusion coating of chromium or chromium-silicon to 400-series stainless steel components.
Accordingly, a diffusion coating process for chromizing or chrome-siliconizing a 400-series stainless steel component includes the steps of mixing a small quantity of calcium fluoride with the chromium or chromium-silicon coating element and ammonium chloride activator salt prior to using one of the prior known processes for applying the diffusion coating. The improved diffusion coating mixture may be applied using the blanket method or that of U.S. Pat. No. 5,912,050 or other known diffusion coating methods.
In a further embodiment, small amounts of silicon powder are also added to the coating mixture.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which a preferred embodiment of the invention is illustrated.